Growth of organisms in continuous culture in a chemostat is being used as a means of studying plaque ecology under controlled environmental conditions. The bacterial cell surface is particularly sensitive to phenotypic change under different growth conditions, and the chemostat enables the study of the effect of changing one parameter - generation time, pH or nutrient - while keeping other factors constant. Strains of streptococci and lactobacilli are being grown under conditions relevant to those in the oral cavity, namely generation times of 1-14 hours, pH values between 5.5 and 7.5 and with either glucose, fructose or sucrose as the limiting carbohydrate or with limiting nitrogen. The surface properties of the organisms are then studied by microscopic, immunological, biological and enzymic techniques, with particular attention being given to the adherence properties. The properties of whole organisms are compared with those of extractable surface components and extracellular components. In addition to continuing studies on cell wall antigens and lipoteichoic acid, particular attention will be given to membrane proteins and to extracellular proteins, lipids and polysaccharide. Parallel studies will concern two other aspects of plaque ecology, first the extent to which changes occur in the cell wall components of Streptococcus mutans strains that have undergone antigenic variation or "drift", and secondly the effect of growing streptococci or lactobacilli in mixed culture with Veillonella on the surface and extracellular components of each organism. These studies should lead to a clearer understanding of the effect of the environment on cariogenic bacteria - an important factor in understanding their role in dental caries, in defining methods for their identification and in developing a caries vaccine.